This invention relates to an ultrasonic motor which utilizes an ultrasonic vibration as a driving power source.
Japanese laid-open Pat. publication No. 63-73887 published on Apr. 4, 1988 discloses an ultrasonic motor which rotates a rotor by utilizing an ultrasonic vibration on a stator. In this type of the ultrasonic motor, a vibration energy on the stator is transmitted to the rotor frictionally in order to rotate the rotor. Desirably, the rotor should have a stable contact with the stator.
FIGS. 4 and 5 herein show a conventional ultrasonic motor. In this type of the ultrasonic motor, a pressure is maintained properly by a disc spring (53) in an effort to obtain a stable contact between the rotor (52) and the stator (51).
While the rotor (52) is rotating, there is applied to the rotor (52) an oscillating pressure with high frequency from the stator (51). Then the rotor (52) may vibrate with a normal component and a tangent component as shown in FIG. 5. At this time, the vibration on the rotor (52) has the same frequency as the vibration on the stator (51). As long as the vibration on the rotor (52) has the same frequency as the vibration on the stator (51), increases in amplitude of the latter are transmitted to the former, so that an output of the ultrasonic motor increases according to the increase in an electric power supplied to the stator (51).
However, it has been found that after the supplied electric power exceeds a certain threshold value, the output of the ultrasonic motor does not increase in accordance with increases in supplied electrical power. Under such condition, a principal vibration begins to be generated in the rotor (52), which is of a different frequency than that of the stator (51). The vibration of the rotor (52) which still has the same frequency as the vibration of the stator (51) is of less amplitude (i.e., less output) than the latter.